Interfacial bonding of polymeric laminae in the solid state

ABSTRACT

The invention relates to chemically bonded laminated structures and their production from oriented polymeric laminae. The bonds between the contacting lamina are formed by chemical reaction across their interface involving constituent molecular chains of each laminae. Three methods have been found for producing the bonds: (a) bringing the surfaces of the lamina in intimate contact to within 10° C. and 100° C. below the melting temperature of the surfaces; (b) interposing a coupling agent for the polymer molecules of the intimately contacting surfaces of each member into their interface and heating the laminate to a temperature effective to cause reaction of the coupling agent with the molecules of each; and (c) interposing a catalyst to promote chemical reaction of the molecules of each laminae with the molecules of intimately contacting lamina into their interface and heating to a temperature effective to cause reaction. 
     Structures bonded may be from film sheets, strips or bands or they may be other configurations that have suitable surfaces for that intimate contact required.

The Government of the United States of America has certain rights tothis invention pursuant to National Science Foundation Grant No.INT-8206731.

This application is a continuation of application Ser. No. 553,106,filed 11-18-83 now abandoned.

BACKGROUND OF THE INVENTION

Interfacially bonded structures or laminations fabricated from two ormore oriented organic polymer films, sheets or strips in recent yearshave found widespread use in a variety of applications by virtue of thehigh level of mechanical properties that can be imparted to thesecomponents by orientation. Further exploitation of increasingly highlevels of properties, however, has been hampered to some extent bycertain limitations imposed by production methods and certain inherentcharacteristics of the materials themselves.

In this regard, biaxially oriented films of a good balance of propertiescan be produced, but in an effort to make biaxially oriented films ofvery high tensile strengths by sequential drawing, the high strengthattained in the first direction draw is diminished by the seconddirection draw. Uniaxially oriented films have a very high level oftensile strength in the direction of orientation, but strength in theorthogonal direction is very low. Such films (especially those which areheat set) can fibrillate so easily that utility, even in applicationsprimarily dependent upon the tensile strength in one direction, isimpaired.

Cross-lapped structures of uniaxially oriented films, with the directionof orientation of adjacent layers at an angle to each other to utilizethe very high tensile strengths of this type film have met with onlylimited success. This limitation is primarily caused by the inability tomake fully satisfactory interlaminar bonds, the poor bonding resultingin delamination failures of the laminated structure.

Highly oriented films of condensation polymers such as polyethyleneterephthalate (PET), are very poorly receptive to adhesives; only a veryfew complex costly adhesives can be used, and only after pretreatment ofthe film to render it more adherable. Adhesives commonly used,furthermore, have very low strength in bulk and in thin layer incomparison to the oriented polymer. Adhesively bonded structures,accordingly, cannot make optimum use of the potential of cross-lappedstructures of uniaxially oriented films.

Likewise, fusion bonding of cross-lapped structures of uniaxiallyoriented film is less than satisfactory in capitalizing on the potentialof such structures. In thermoplastic films of major commercialimportance, such as representative condensation polymers PET,poly(butylene terephthalate) (PBT), the aliphatic polyamides (e.g.,nylon 6 and nylon 66) and polyolefin addition polymers, fusion destroysorientation. In the art, U.S. Pat. No. 4,384,016 discloses fusionbonding of polymers which form liquid crystals in the melt. These aredescribed as wholly aromatic condensation polymers. Any orientationimparted to these polymers, usually formed in melt drawing, is not onlyretained upon melting, but survives upon returning to the solid state.Polymers to which the present invention is directed are understood tonot retain their oriented structure upon melting, but at best formcollapsed coils. Accordingly, for the polymers covered by the presentinvention, fusion bonding is generally unsatisfactory.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide alaminate with an improved bond and a process for its formation betweensurfaces of solid, molecularly oriented polymers, with no substantialloss of molecular orientation.

It is a further object of the present invention to provide a bondedlaminar structure and a process for its formation from orientedcondensation polymer members not suitable for fusion bonding.

It is another object of the present invention to provide a bondedlaminar structure and a process for its preparation from orientedpolymeric members in which satisfactory bonds cannot be formed by anadhesive element.

These and other objects of the invention are achieved by chemicalinteraction across the interface of the laminae between molecules ofeach lamina. These interactions are accomplished in the solid state bythree optional means:

(a) by postcondensation, such as occur in solid phase polycondensation,or by an interchange reaction between molecules of adjacent laminae;

(b) by interposition of coupling agents for polymeric chains,constituents of the laminae, into their interface; and

(c) interposition of catalysts for interchange or condensation ofconstituent molecular chains into the interface of the laminae.

The particular conditions, heating times and temperatures and the use ofagents to facilitate the reactions are a matter of choice depending uponrequirements relating to the intended use of the joined product. Theessence of the invention is that durable delamination-resistant bondscan be obtained without fusion, which would be accompanied by asignificant loss of orientation, or the use of bonding materials whichare adhesives per se.

DETAILED DESCRIPTION

In a broad sense, the present invention is directed to the establishmentof chemical linkages between contacting solid members of a structure.These linkages involve the substances of the materials joined; they neednot be between elements of identical compositions but only that thematerials joined have polymeric entities capable of chemical reactionwith polymeric entities of the other. For example, polyesters may bejoined to polyamides by means of an amide linkage between an amide oramine group of the polyamide and a carboxyl group of the polyester. Itis not necessary that the members of a bonded structure be of the samethickness or have other properties in common; one member may be orientedand the other need not be, depending upon the properties desired for aspecific use. Structures other than film may be bonded provided theyafford surfaces for intimate contact.

The essence of the present invention is that effective bonding oflargely crystalline materials is accomplished in the solid state, belowthe melting temperature. This provides a means for bonding with theirown substance of highly crystalline materials.

In view of the relatively inert nature of surfaces of materials such asoriented PET, it would not be expected that chemical bonding betweenstructures could occur in the solid state. Solid phase polymerization atelevated temperatures wherein further condensation of oligomers orpolymers of finely divided solids in fluidized beds is widely used. Sucha process would be inoperable if the chemical reaction extended beyondthe boundaries of separate particles; bonding and agglomeration would beprohibitive. In the present invention means for extension of polymerreactions across the interfacial boundary of separate bodies and theformation of chemical bonds between two bodies have been found.

Requirements for such reactions and bonding are:

(1) The surfaces of the bodies to be bonded must be brought into veryintimate contact, preferably under pressure; and

(2) The contacting surfaces must be subjected to conditions for chemicalreaction between molecular species of each surface.

The first of these requirements is most readily met by urging thesurfaces together under mechanical pressure. Vacuum may be applied toexclude trapped air or to eliminate oxygen or volatile by-products ofthe reaction.

To meet the second requirement, several alternative means have beenfound:

(a) Heating the surfaces, before or after bringing them into contact, toan elevated temperature approaching but below the melting point,representative temperatures being 10°-100° C. below the melting point;

(b) Application to at least one of the surfaces to be bonded of acoupling agent effective to interact with groups on the polymeric chainof each of the bodies to be bonded and thereafter contacting thesurfaces which are brought to a temperature effective to initiateinteraction of the coupling agents and the polymeric chains; or

(c) Applying a catalyst for condensation or interchange reaction to atleast one surface of a pair of surfaces to be bonded and bringing thesurfaces into contact while at a temperature sufficient to activate aninterchange or condensation reaction. For bonding polyesters, forexample, but not limiting, poly(ethylene terephthalate) andpoly(butylene terephthalate), zinc acetate, antimony oxide and germaniumoxides in catalytic quantities are effective.

The choice from these methods depends upon the type and strength of thebond desired and production methods and facilities to be employed.

EXAMPLE 1

To establish that chemical bonding occurs across the interface betweentwo contacting surfaces of films, samples of poly(ethyleneterephthalate) were examined by the first of the aforesaid methods, thethermal or transreaction method, as follows.

Film samples employed were commercial poly(ethylene terephthalate) (PET)(Mylar® polyester film, supplied by E. I. du Pont de Nemours and Co.,Inc., Wilmington, Delaware). This film had a thickness of 0.4 mm, wasbiaxially oriented and heat set to a crystallinity of about 35%. It hada melting point of 248° C., as determined by means of a Du PontDifferential Thermal Analyzer, Model 990, in a nitrogen atmosphere usinga 5 mg sample of polymer heated at a temperature rise of 20° C./minute.Melting points were interpolated from the dH/dT vs. temperature curves.The molecular weights of the polymer in the samples were determined bygel permeation chromatography (GPC) in meta-cresol as a solvent. Asreceived, the polymer in the film had a weight average molecular weightof 50,700.

To avoid sticking of the polymer sheets, (cut into strips of a typicalwidth of 0.5 to 1 cm) as a result of partial melting and to assure thatadhesion between strips would be due to chemical bond formation with aminimum of diffusion bonding, before bringing them into bonding contactthey were annealed to increase crystallinity to as high a value aspossible. It had previously been observed that annealing increases themelting point, so the samples could be heated to a temperature above theinitial melting point by simply bringing the film up to a temperaturenear the melting point in a slow, step-wise manner.

For this annealing the samples were clamped in a vise with Teflon®fluorocarbon film to prevent sticking to the vise. Heating (stepwiseincrease from room temperature to just below the melting point) was for6 hours in a vacuum following a nitrogen purge. After annealing, thesamples had a crystallinity of 62%.

Samples, after annealing, for the thermally induced transreaction andpostcondensation method were prepared in the following way. Two stripswith a length of 3 cm were placed one over another with an overlap ofbetween 1 cm and 2 cm. The samples were placed between two polishedsteel plates (with Teflon® sheets over the metal surfaces) and pressedtogether in a drill press vise.

The heat-bonding process was conducted in an evacuated (oil pump) oven,previously purged with nitrogen. The samples were heated to 240° C. fortimes of 10, 20, 30, and 40 hrs.

This heat-bonding process resulted in bonding of all samples,accompanied by increases in melting point, degree of crystallinity andmolecular weight. Tests with an Instron tensile tester for strength tobreak showed that when the overlap area (length) was large, the unbondedparts of the film of most samples broke before the bond failed bydelamination under shearing stress. Conversely, when the overlap areawas small, failure occurred largely by the shear failure of the bond. Acritical bond length is computed from the intermediate case in whichapproximately 50% of the specimens failed by each mechanism. From theseveral samples bonded at several heating times in which the bondfailed, representative values of the critical shear stress and thecritical contact length were calculated. Critical contact length is thelength of bond for which 50% of the specimens fail by interfacialdebonding. In other words, the stress to break multiplied by thecross-sectional area of the film is equal to the shear stress todelaminate multiplied by bonded area. Since the width of each is thesame, the relationship becomes: the stress-to-break multiplied by thefilm thickness is equal to the stress to shear times the critical bondlength. The critical bond, then, is inversely related to the extent ofbonding. Values these PET samples presented in Table I indicate a fairlyconstant (8.0 to 12.2) level of bond formation between laminae over arange of heating times.

                                      TABLE I                                     __________________________________________________________________________                                      Critical                                                                 Stress to                                                                          Bond                                        Sample                                                                             Bonding Time                                                                          M.P.                                                                              Crystallinity                                                                        MW   Break                                                                              Length                                      Number                                                                             (hrs.)  T °C.                                                                      %      × 10.sup.-3                                                                  Kg/cm.sup.2                                                                        mm                                          __________________________________________________________________________    Annealed                                                                            0      252 62     35.2 116  --                                          1    10      258 64     77.7 259  10.4                                        2    20      262 64     62.6 201  8.8                                         3    30      260 66     73.4 180  8.0                                         4    40      268 57     91.6 170  12.2                                        __________________________________________________________________________

These data are indicative of bonding resulted since highly crystallinematerials have few amorphous molecules with a mobility sufficient todiffuse across the boundary and bond. In practical applications, theannealing step which causes excessive crystallization and loss oforientation, might be omitted since diffusion bonding which could occurat lower crystallinity levels would not significantly harm or interferewith chemical, transreaction bonding by this process. The bond formed bythe combination of chemical accompanied by other types of bonding couldgenerally be more effectual than either alone, but chemical bondingcould supplement the other types.

EXAMPLE 2

Alternative means, with coupling agents or by catalyst enhancement,could not only provide evidence of chemical bonding but could offer anattractive means of achieving chemical bonding with somewhat less severeconditions. Examples of the efficacy of these can be demonstrated bycomparing the effect of bonding in the presence and absence of theseagents. The time and temperature of heating, the pressure urging thesurfaces into contact, ambient atmosphere, and the quantity and thecomposition of catalysts or coupling agents are technological variablesand may be selected as appropriate by one skilled in the art.

Samples of Mylar® polyester film similar to those the foregoing thermalmethod Example 1 were employed, but without annealing. This film was cutinto strips 0.5 to 1 cm wide, 7-10 cm long; the ends of two strips wereoverlapped for a short distance to allow bond failure before filmbreakage. A control sample had the bare surfaces in contact at theoverlap, samples for the coupling agent tests had a small quantity,approximately 100 mg and 10 mg, of the respective agents, spread overthe surface of one strip to be contacted with the other strip. Theoverlapping areas of the respective strips were urged into intimatecontact by being pressed together in Teflon® protected jaws of a vise.They were heated in a nitrogen purged evacuated (oil pump) oven for 6hours at a temperature of 240° C. After cooling the samples werequalitatively tested by hand pulling for evidence of bonding withresults as follows:

                  TABLE II                                                        ______________________________________                                        BONDING PET BY COUPLING AGENT ACTION                                          Sample              Bonding Behavior                                          ______________________________________                                        Control             No adhesion                                               Coupling Agent (1)  Good adhesion (5)                                         Coupling agent (2)  Good adhesion                                             Coupling agent (3)  Good adhesion                                             Coupling agent (4)  Good adhesion                                             ______________________________________                                         Notes:                                                                        (1) Pyromellitic acid                                                         (2) Benzophenone tetracarboxylic acid                                         (3) Oxalic acid                                                               (4) Malonic acid                                                              (5) Good bond adhesion indicated by tough bond which in a few cases only      could be separated by severe twisting.                                   

EXAMPLE 3

Further examples of coupling poly(ethylene terephthalate)andpoly(butylene terephthalate) with the agents indicated in Example IIwere conducted with the stress to shear determined with an Instrontensile tester.

Biaxially oriented, heat set PET film (Du Pont "Mylar" polyester film)of 0.4 mm thickness was bonded, as in Example 2 but without annealingthe film before bonding. Results are indicated in Table III. Filmsamples, 0.6 to 0.95 cm in width with a 1.2 cm overlap were urged intointimate contact in a vise and heated to 240° C. for 6 hours. The filmswere not annealed before bonding so embrittlement was insignificant.Coupling agents identified by numerals are as indicated in Table II. Drycoupling agents or in tetrahydrofuran (THF) solution were spread on thesurface to be bonded.

                  TABLE III                                                       ______________________________________                                        COUPLING               SHEAR STRESS                                           AGENT         STATE    Kg/cm.sup.2                                            ______________________________________                                        (1)           Solid    10.9                                                                 Soln.    14.6                                                   (2)           Solid    13.5                                                                 Soln.     7.1                                                   (3)           Solid    Film Failed                                                          --       --                                                     (4)           Solid     8.8                                                                 Soln.     5.9                                                   Control       --       None                                                   ______________________________________                                    

EXAMPLE 4

Samples of PBT film 0.5 to 1 mm in thickness, with other dimensions assimilar PET samples in Example III were comparably overlapped and urgedinto contact in a vise. They were heated to 195° C. for 15 hours. Alsoincluded were two samples of ester interchange catalyst, zinc acetate,as indicated in Table IV. All agents were in the solid state.

                  TABLE IV                                                        ______________________________________                                        AGENT OR CATALYST                                                                              SHEAR STRESS (Kg/cm.sup.2)                                   ______________________________________                                        (1)              Film Failed                                                  (2)              10.75                                                        (3)              4.27                                                                          2.99                                                         (4)              2.98                                                         (1) + Zn(OAC).sub.2                                                                            2.69                                                         Zn(OACO.sub.2    3.17                                                         ______________________________________                                    

EXAMPLE 5

Two nylon 12 (--(CH₂)₁₁ --CONH--) film strips melting point 194° C.,thickness approximately 0.5 mm, were subjected without prior annealingto thermally-induced bonding at 140° C. for 1 hour, as in Example 1. Thebonded samples were subjected to a tensile stress to determine bondstrength. The unbonded portions of the strips necked and elongatedwithout failure of the bonds. These very strong bonds were formed at 54°C. below the melting temperature.

EXAMPLE 6

Samples of oriented poly(ethylene terephthalate), thickness ca 0.05 mm,coated on overlapping areas with the monomer di(hydroxyethyl)terephthalate, pressed in a vise and heated to 150° C. for 30 minutesshow good bonding and no significant loss of orientation as indicated bytensile strength.

EXAMPLE 7

Samples of polyamide film, nylon 66 (m.p. 265° C.), thickness ™0.2 mmwere examined for bonding by the method of Example 1 except that beforeurging them into contact for bonding, they were not annealed. Sampleswere tested for thermally induced bonding (transreaction) betweensurfaces of the polyamide, between the polyamide and PET and between thepolyamide and PBT, all heated for bonding for 6 hours. Significantpreparation and testing details with results are summarized in Table V.

                  TABLE V                                                         ______________________________________                                                            Stress at Stress at                                       Laminate  Bonding   Film Break                                                                              Bond Failure                                    Components                                                                              Temp. °C.                                                                        kg/cm.sup.2                                                                             kg/cm.sup.2                                                                            Notes                                  ______________________________________                                        Nylon 66 -                                                                              248       --        8.5      (1)                                    Nylon 66                                                                      Nylon 66 -                                                                              240       --        12.5     (1)                                    Nylon 66                                                                      Nylon 66 - PET                                                                          248       35        --       (2)                                    Nylon 66 - PBT                                                                          210       55        0        (2)                                    ______________________________________                                         Notes:                                                                        (1) Bond failed before film breakage.                                         (2) Film broke before bond failure.                                      

The foregoing examples are illustrative of chemical bond formationacross the interface of intimately contacting bodies. Thethermally-induced bonding method is drastic, primarily with itsannealing to high crystallinity to minimize the presence of amorphousmolecules to put emphasis on the role of chemical bonding. A detrimentaleffect on orientation probably resulted, although the samples in thevise were under restraint which could reduce relaxation of orientation.But it showed selfbonding without fusion of oriented polycondensates.Operators skilled in the art could employ the adjuncts of couplingagents and catalysts, along with flash heating (flame) of the surfacesat the junction as films are brought together in a nip rolls for bondingto minimize heating the bulk of the film and accelerate bonding. Suchwould help retain orientation.

Examples show applicability of the invention not only to bonding ofsurfaces of the same composition, but also of different compositions.The method includes chemical bond formation with representative aromaticaliphatic polyesters such as poly(ethylene terephthalate), poly(butyleneterephthalate), and poly(1,4 cyclohexane dimethylene terephthalate).Polycarbonate and cross-linked polyesters used for potting resins can bebonded by this method. Polyamides, principally the aliphatic polyamides,but also the all-aromatic polyamides can be bonded. Typically, nylon 6,nylon 66 and nylon 12 are useful.

Structures adapted to the present invention are not limited to films,which for convenience were employed in the examples to illustratebonding without fusion, but the process is adapted to other structureshaving surfaces for the required contact. These structures to which theinvention is adapted include straps, bands, billets, tubes, pipes, androds. Pipe structures and cable wraps with cross-lapped strip windingsof uniaxially oriented film bonded at the laps are tightly resistant tobursting and are a major use for the method. Welding of composite sheetsand rods, including rods such as fiber-reinforced cross-linked polyesterfishing rods are of major interest. Rib-reinforced panels, a pair ofadhered sheets formed with ribs on one sheet for beam-like strength,could advantageously employ the present invention. Such structures aremade by positioning the sheets between a pair of heated platens, atleast one having cavities for forming ribs as air is drawn into thecavities through the interface of the sheets. After rib formation thesheets are urged together and bonded. Adhesives give problems withpremature sticking. Chemical bonding according to this inventionobviates this and permits the use of oriented sheets.

Chemical bonding of other than condensation polymers is illustrated inco-pending U.S. patent application Ser. No. 553,103, filed 11/18/83, nowU.S. Pat. No. 4,575,470, with a common assignee and two of the threeco-inventors common to the present application. The invention disclosedin that application relates to chemical bonding of polyolefin laminae bythe action of cross-linking initiators. That application is incorporatedherein by reference.

We claim:
 1. A process for production of a bonded laminar structure ofcrystalline, oriented linear polyester laminae, the laminae being of acomposition which loses its crystallinity and orientation upon melting,the bonds being chemical bonds formed by solid state reaction betweenmolecules of each lamina across the interface of contacting surfaces oflaminae by the steps in sequence comprising:application to at least oneof two adjacent surfaces to be bonded of an agent effective in theabsence of externally applied radiation to cause chemical reactionbetween molecules of polymers of adjacent laminae; bringing the surfaceshaving said agent on at least one surface to be bonded together to forma laminate; urging the surfaces to be bonded into intimate contact withpressure; and heating the laminate while being urged into contact to atemperature from 10° to 100° C. below the melting point of the polyesterof the laminate to form chemical bonds.
 2. The process of claim 1wherein a vacuum is applied to the laminate during heating to formchemical bonds.
 3. The process of claim 1 wherein the linear polyesteris selected from polyethylene terephthalate and polybutyleneterephthalate.
 4. The process of claim 1 wherein the agent effective tocause reaction is selected from monomeric species of the polyester ofthe laminae.
 5. The process of claim 4 wherein the monomeric species isdi(hydroxyethyl) terephthalate.
 6. The process of claim 1 wherein theagent to cause reaction is selected from condensation catalysts andester exchange catalysts.
 7. The process of claim 6 wherein the catalystis selected from zinc acetate and antimony oxide.
 8. The process ofclaim 1 wherein the agent to cause reaction is a coupling agent.
 9. Theprocess of claim 8 wherein the coupling agent is selected frompyromellitic acid, benzophenone tetracarboxylic acid, oxalic acid, andmalonic acid.